Generally, in vehicle side body structures, a roof rail section, a slanting front pillar section, a lower front pillar section, a side sill section and a center pillar section are formed of plates (or plate materials), such as cold rolled steel plates, and these plates are sequentially fixedly joined together to construct an outer side panel unit. Namely, first, the roof rail section, slanting front pillar section, lower front pillar section and side sill section are fixedly joined together, and then, the center pillar section is fixedly joined to the thus joined-together roof rail section, slanting front pillar section, lower front pillar section and side sill.
With such vehicle side body structures, where all of the plates except for one plate are first fixedly joined together and then the one plate is fixedly joined to the joined-together plates, dimensional accuracy of the individual plates need not be enhanced more than necessary. One example of such vehicle side body structures is disclosed in Japanese Patent Application Laid-Open Publication No. HEI-8-243770 A (hereinafter referred to as “the relevant patent literature”).
According to the disclosure of the relevant patent literature, in manufacturing the outer side panel unit, a plurality of blank plates are sequentially laser-welded to form a frame structure, then a blank plate for forming the center pillar section is spot-welded to the frame structure, and then the structure is pressed to finish an intended product.
In recent years, there has been an increasing need for the outer side panel unit to have an increased strength for sufficiently bearing a lateral collision load and have a reduced weight, and thus, a high-tensile steel plate is used for reinforcing the center pillar section in many cases. However, if such a high-tensile steel plate is used for forming the roof rail section of the outer side panel unit disclosed in the relevant patent literature, small bends cannot be formed by press forming. Thus, the technique disclosed in the relevant patent literature cannot achieve an improved outer appearance by reducing a gap between the roof rail section and a door. Even when a high-tensile steel plate is used for forming the center pillar section, the center pillar section merely overlaps the roof rail section, and thus, a lateral collision cannot be dispersed efficiently.
Further, because the center pillar section needs to have a sufficient strength for bearing a side or lateral collision load, it is desirable that a high-tensile steel plate be used for forming the center pillar section. However, the center pillar section is of a shape requiring deep drawing work, and thus, if a steel plate of a tensile strength equal to or greater than 590 MPa is used for forming the center pillar section, then there may undesirably occur cracks in the center pillar section during press forming.
Further, in the vehicle side body structure disclosed in the relevant patent literature, the frame structure, functioning as frames defining a front door opening of the vehicle, is formed by joining together five plates. Of the outer side panel unit of the vehicle side body structure, a front frame (i.e., outer front pillar section) is formed by fixedly joining two plates. An upper frame (i.e., outer roof rail section) and lower frame (i.e., outer side sill section) are fixedly joined to the front frame. Then, a center frame (outer center pillar section) is joined at its upper end portion to the upper frame in overlapping relation thereto and joined at its lower end portion to the lower frame in overlapping relation thereto. Such arrangements can facilitate the aforementioned joining without requiring dimensional accuracy of the individual plates more than necessary.
However, if a high-tensile steel plate is used for forming the center frame (outer center pillar section), there may undesirably occur cracks in corners of the center frame (outer center pillar section) after plastic forming of the center frame. Further, if a higher-tensile steel plate, such as a steel plate having a tensile strength of 1,000 MPa, is used for the outer center pillar section with a view to achieving a reduced weight and increased strength, greater pressing force would be required for plastic forming, so that a pressing machine of a greater size would be required.
Thus, there has been a demand for a structure which does not require great pressing force, can secure a sufficient strength of the outer center pillar section and can prevent cracks from occurring in the outer center pillar section even if corners of the outer center pillar section are formed in small bending radii.